fix: update_column_spec typo in TablesClient docstring

[na0fu3y]

The docstring contains the non-existent function update_column_specs, and fixed it to update_column_spec which I think is more appropriate.

The code cause AttributeError: 'TablesClient' object has no attribute 'update_column_specs'.

Fixes #17 🦕

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[na0fu3y]

@googlebot I signed it!

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